Adaptive immunity plays a critical role in eradicating viral infections. In 2018, this project has aimed to study in vivo mechanisms of virus - host interactions and to better understand what constitutes effective primary and memory B and T cell responses to virus infection. Our first research topic examines the consequences of partial or incomplete immunity. Due to the constant antigenic evolution of influenza A viruses (IAV), the specificity of existing antiviral immunity in the human population is typically at best only partially cross-reactive to the infecting virus. Uncertainties about future circulating strains further results in partial immunity through the selection of best guess strains for annual vaccines that are not perfect matches. The existence of partial B cell and CD8+ T cell immunity raises several important questions including: Are all immunodominant B and T cell specificities equally effective at protecting against infection? Can partial immunity result in the amplification of ineffective immunodominant responses at the expense of more effective specificities? Does partial immunity enhance virus evolution? A second topic of our studies aims to better understand and predict the best effector functions required for protective antiviral memory responses. Specifically, can the most effective specificities and functional features of T and B cell responses be defined? How can these be optimally induced as immunodominant responses through vaccination? These basic studies are needed to improve the rational design of seasonal vaccines and interventions against influenza virus infection. They are important to elucidate what can generate the most effective immunity against IAVs and the potential impact that vaccination may have on virus evolution and viral pathogenesis. In 2018, interactions between programmed death-1 (PD-1) and its ligands were examined to determine the effect on immunodominance and the breadth and effectiveness of specific CD8+ T cell responses.